Dielectric shield for ultrahigh frequency thermionic tubes



Dc. 19, 1967 HIROMI MURAKAMI ETAL 3,

. DIELECTRIC SHIELD FOR ULTRAHIGH FREQUENCY THERMIONIC TUBES Filed Feb. 4, 1965 Inventor H- M RAKAM I A Home United States Patent 3,359,446 DIELECTRIC SHIELD FOR ULTRAHIGH FREQUENCY THERMIONIC TUBES Hiromi Murakami and Yosihiro Morizumi, Tokyo,

Japan, assignors to Nippon Electric Company,

Limited, Tokyo, Japan, a corporation of Japan Filed Feb. 4, 1965, Ser. N 0. 430,422

' Claims. (Cl. 313-242) ABSTRACT OF THE DISCLOSURE The invention is a device to reduce temperature build up and consequent failure in electronic tubes where a large number of high-energy electrons may strike the insulator. It is comprised of a band of conducting material around the inside of a tubular dialectric shield. The band may further be connected to one of the electrodes as a means of disposing of charges thereon.

This invention relates to ultrahigh-frequency disc-sealed thermionic tubes such as ultrahigh-frequency transmitter tubes, klystron, etc.

Ultrahigh-frequency disc-sealed thermionic tubes generally are provided with parallel disc-shaped electrodes, such as the anode and grid electrodes. These electrodes are spaced by an insulator tube formed of glass, ceramic, or other electrically insulating material. This insulator also serves as a portion of the vacuum envelope. In this arrangement the insulating material emits large quantities of secondary electrons, when bombarded by electrons. The inside surface of the insulator tube therefore reaches an extraordinarily high electric potential at those portions thereof which are subjected to this electron bombardment. These high-potential portions in turn at:- celerate stray electrons which have missed the anode. As a result, the insulator is further heated by the bombardment of the thus accelerated electrons. As a result of this cycle, the insulating material is often damaged by the thermal stress produced by the local intense heating, particularly if the anode potential is high for high-power operation or for pulse operation or if the anode potential is high because of abnormal operation. Furthermore, frequently when the thermionic tube is operated at ultrahigh frequencies, the multipaction effect of the bombarding electrons and the presence of ultrahigh-frequency electric fields will accelerate breakage of the insulator tube.

A first conventional method for overcoming these deterimental effects has been to apply a material which has a small secondary electron emission onto the inside surface of the insulator tube. A second known method requires that a metal tube for receiving stray electrons be arranged concentrically within the tube to prevent these electrons from reaching the inside surface of the insulator tube. In the first method mentioned above, the substance evaporated or sputtered from the electrode during operation of the tube deposits on the applied surface to reduce its ability to prevent secondary electron emission. As a result, in due course, the interelectrode insulation will be damaged. This is particularly serious for ultrahigh-frequency tubes in which the insulator tube must be positioned in close proximity of the electrodes. In the second method mentioned above, it should be noted that the metal tube placed around the electrodes not only increases the interelectrode capacity but also increases the currents induced therein during high-frequency operation. As a result, there is an increase in the inductance which in turn degrades the high-frequency performance of the thermionic tube. This degradation is proportional to the increased interelectrode capacity.

An object of this invention is therefore to provide an improved ultrahigh-frequency disc-sealed thermionic tube in which the interelectrode insulation remains undamaged even when subjected to electron bombardment.

Another object of this invention is to provide a thermionic tube in which the interelectrode insulation does not substantially decrease during operation and in which the interelectrode capacity and spurious inductances do not substantially increase during operation and which is not damaged by severe multipaction effects.

According to this invention, an improved ultrahighfrequency disc-sealed thermionic tube is provided in which a shield is disposed within the member spacing two adjacent parallel generally disc-shaped electrodes. This shield electrically insulates the electrodes from each other and also maintains the vacuum within the tube. The shield is formed of an insulating material which is held with an end thereof attached to one of two electrodes and with the other end thereof extended near to the other of the two electrodes. The shield which is a cylindrical tube in shape, is provided with a band of electrically conductive material on at least that portion of its inside surface which is bombarded by the stray electrons. The inside surface of the shield tube may further be provided with a strip of electrically conductive material connecting this band to one of the electrodes and preferably to the electrode having the lower electric potential, which electrode is also preferably the one to which the shield tube is attached. Alternately, a plurality of bands of electrically conductive material may be formed on the inside surface instead of a single band.

It will be readily understood from the above, that the shield prevents stray electrons from bombarding the inside surface of the tube member (which would eventually cause breakage of the thermionic tube) and maintains the interelectrode insulation even when it is.covered with evaporated or sputtered anodic substances. This arrangement also allows the tube member to be made completely of an electrically insulating material having an annular portion of small width. Inasmuch as electrically conductive material generally emits less secondary electrons than electrically insulating material, the conductive band serves to not only reduce the undesired electric potential generated by the stray electrons striking against the inside surface of the shield but also to reduce multipaction effects. Because of the small total area of the electrically conductive portion, hardly any appreciable increase appears in either the interelectrode capacity or the spurious inductance. This is so even when the tube is operated at ultrahigh-frequencies. In addition, the cantilever support of the shield prevents the appearance of any thermal stress within the shield tube. It the shield is made of beryllia ceramic or other highly thermo-conductive ceramic, then it will also be protected against temperature rise. The strip of electrically conductive material, discharges the positive charge which would otherwise accumulate on the inside surface of the shield tube even when the band is covered with the evaporated or sputtered anodic materials.

The above-mentioned and other features and objects of this invention and the means for attaining them will become more apparent and the invention itself will be best understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a perspective view of an embodiment of this invention; and

FIGURE 2 illustrates a fragmentary perspective view of another embodiment of this invention.

Referring to FIG. 1, the tube of this invention includes a generally disc-shaped anode 11 which also serves as a portion of the vacuum envelope. Anode 11 has an axial extension 13 for receiving a radiator, not shown, for radiating the heat generated by the tube. A generally disc-shaped grid electrode 15 is disposed parallel to the anode 11. A cathode 17 is positioned opposite the anode 11. A tube member 19 spaces the anode 11 from the grid electrode 15 and also electrically insulates these members from each other and forms a portion of the vacuum envelope. The techniques for assembling the tube as indicated above are well known in the art. According to this invention, however, a shield in the form of tube 21 is provided and formed of an electrically insulating material. Tube 21 is held concentrically within the tube member 19 with an end thereof metallized and brazed. Shield tube 21 is attached at one end thereof to grid electrode 15 by the use of glass or any other refractory cement, or fixed rigidly to grid 15 in any conventional manner. The other end of tube 21 extends slightly beyond the main surface of the anode 11 which faces the grid electrode 15. In addition, a band 23 of electrically conductive material is formed on tube 21 by metallizing sputtering, spraying, painting, or otherwise. This band 23 is formed around the inside surface of the shield tube 21 preferably at that circumferential portion thereof which is most severely bombarded by the stray electrons. The shield tube 21 may be made of beryllia ceramic, alumina ceramic, or other ceramic which preferably has excellent thermal conductivity. The conductive material for the band 23 may be nickel, titanium, carbon, .or other material having low secondary electron emission characteristics. It will be clear that this embodiment has the technical merits set forth hereinabove. The tube is then mounted, evacuated and sealed on a base, such as a sealing ring 35, in a conventional manner. The tube is energized, for example, by clip-on leads (not shown) which clip on for example, to each disc electrode 11 and 15 and the cathode 17.

Referring now to FIGURE 2, another embodiment of this invention is illustrated therein which further includes a strip 25 of electrically conductive material formed on the inside surface of the shield tube 21 between the band 23 and the grid electrode 15. This strip 25 may be made of any electrically conductive material and placed on tube 21 in a manner similar to that used for the band 23. This embodiment is particularly advantageous when shield tube 21 is to be disposed near the main portions of the electrodes 11 and 15 and when any of the electrodes 11, 15, or 17 are made of material which will evaporate or sputter. The thus evaporated or sputtered substance emits larger quantities of secondary electrons than the band 23 and causes the inside surface of the shield tube 21 to be positively charged. This positive charge, however, is bled through strip 25 to the grid electrode 15, thereby preventing the build-up of an extraordinarily high electric potential on said inside surface.

While the illustrated embodiments show the shield 21 attached to the grid 15, it should be understood that the shield 21 can be attached to the anode 11 or disposed in the space between the grid electrode 15 and the cathode 17, if desired. Furthermore, the disc-sealed tube may comprise only the anode and the cathode or include in addition to the anode 11, the grid electrode 15, and the cathode 17.

While we have described above the principles of our invention in connection with specific embodiments, it is to be clearly understood that this description is made only by way of example, and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. In an ultrahigh-frequency disc-sealed thermionic tube the combination comprising: a cylindrical tubular member formed of an electrically insulating material; .a pair of disc-shaped electrodes respectively disposed at and sealed to opposite ends of said member; tubular shielding means, also formed of an electrically insulating material, mounted within said tubular member and having one end thereof attached to one of said electrodes with the other end thereof being adjacent but spaced from the other of said electrodes; at least one band of electrically conducting material disposed on at least a portion of the inside surface of said tubular shielding means; and means for sealing said tube.

2. In the ultrahigh-frequency disc-sealed thermionic tube as set forth in claim 1 wherein .the tubular shielding means is formed of a ceramic material.

3. In the ultrahigh-frequency disc-sealed thermionic tube as set forth in claim 2 wherein the ceramic material is selected from the group consisting of: beryllia and alumina.

4. In the ultrahigh-frequency disc-sealed thermionic tube as set forth in claim 1 wherein the band of electrically conductive material is selected from the group con sisting of: nickel, titanium and carbon.

5. In the ultrahigh-frequency disc-sealed thermionic tube as set forth in claim 1 wherein connecting means are provided for connecting said conducting band to one of said disc electrodes.

6. In the ultrahigh-frequency disc-sealed thermionic tube as set forth in claim 5 wherein the connecting means connects said conducting band to the electrode having the lower potential.

7. In the ultrahigh-frequency disc-sealed thermionic tube as set forth in claim 1 wherein one of said pair of electrodes is an annular anode and the other is an annular grid electrode having an aperture therethrough and wherein a cathode electrode is provided and extends through said aperture of said annular grid electrode.

8. An improved evacuated ultrahigh-frequency discsealed thermionic tube in which a cylindrical tubular insulating member separates a pair of disc-shaped electrodes respectively sealed to the ends thereof and wherein sealing means provide an air tight seal for said tube, the improvement comprising: tubular non-conducting shielding means mounted within said tubular member and having one end thereof attached to one of said disc electrodes with the other end thereof being adjacent but spaced from the other of said disc electrodes; and at least one band of electrically conducting material disposed on at least a portion of the inside surface of said tubular shielding means.

9. An improved evacuated ultrahigh-frequency discsealed thermionic tube as set forth in claim 8 wherein connecting means are provided for connecting said conducting band to one of said disc electrodes.

10. An improved evacuated ultrahigh-frequency discsealed thermionic tube as set forth in claim 8 wherein one of said pair of electrodes is an anode and the other is an annular grid electrode having an aperture therethrough and wherein a cathode electrode is provided and extends through said aperture of said annular grid electrode.

References Cited UNITED STATES PATENTS 1,889,449 11/1932 McCullough 3 l3239 2,178,747 11/1939 Espe 313-813 2,446,554 8/ 1948 Rouy 3 l3240 2,683,236 7/1954 Rawls et al 313-334 2,889,484 6/ 1959 McIlvaine 3 l33 13 3,023,341 2/1962 Kendall et al 313250 JOHN W. HUCKERT, Primary Examiner.

'A. J. JAMES, Assistant Examiner. 

1. IN AN ULTRAHIGH-FREQUENCY DISC-SEALED THERMIONIC TUBE THE COMBINATION COMPRISING: A CYLINDRICAL TUBULAR MEMBER FORMED OF AN ELECTRICALLY INSULATING MATERIAL; A PAIR OF DISC-SHAPED ELECTRODES RESPECTIVELY DISPOSED AT AND SEALED TO OPPOSITE ENDS OF SAID MEMBER; TUBULAR SHIELDING MEANS, ALSO FORMED OF AN ELECTRICALLY INSULATING MATERIAL, MOUNTED WITHIN SAID TUBULAR MEMBER AND HAVING ONE END THEREOF ATTACHED TO ONE OF SAID ELECTRODES WITH THE OTHER END THEREOF BEING ADJACENT BUT SPACED FROM THE OTHER OF SAID ELECTRODES; AT LEAST ONE BAND OF ELECTRICALLY CONDUCTING MATERIAL DISPOSED ON AT LEAST A PORTION OF THE INSIDE SURFACE OF SAID TUBULAR SHIELDING MEANS; AND MEANS FOR SEALING SAID TUBE. 